Various electromagnetic transducers have been proposed for use in sensing position. A traditional approach is the use of optical encoders. However these devices are expensive and prone to dirt contamination unless housed, at added cost. More recently Hall-effect based sensors have become available, dedicated to the task of absolute position sensing. These are exemplified by Austria Microsystems' AS5030 integrated circuit, which measures the angular position of a magnet positioned above. However these sensors are relatively sensitive to DC magnetic fields present in their operating environment, for example due to their proximity to motors and/or the presence of magnets. They are also sensitive to misalignment of the magnet's rotation axis relative to their own central axis, which can cause errors in their reported position. Such misalignment arises through tolerances at manufacture. It is possible to calibrate out the resulting errors, but the resulting calibration time is costly. It is possible to mechanically trim out the misalignment, but the trimming step is costly. It is possible to tighten the tolerances, but this usually results in the need for more expensive components.
Inductive sensors such as those described in U.S. Pat. No. 6,522,128 (the contents of which are incorporated herein by reference) overcome the problem of DC magnetic field sensitivity by operating with AC fields. Many of these known inductive sensors use excitation and sensor coils that are inductively coupled, in use, to a resonator coil. To keep manufacturing costs down, the coils, including the resonator coil, are manufactured from conductive tracks mounted on printed circuit boards. This can result in relatively low Q factor and hence poor signal levels and hence poor signal to noise for a given drive power.